Hmm, I'm having 2nd thoughts up leveling up the skills. You might actually have to use the same units to level it up... In any case, if possible, can you guys fuse different type of golems to one golem? (so if you're fusing light golem, don't use light golem during fusing.) And please do tell us if the skill ever levels up.
Well I got a Golem. So I'll give it a shot. I'll fuse some Golems into my 3* Earth Golem and see if his skill Level goes up.
Yeah, I'm 99.99% sure that you have to fuse a Hero with the same active skill name to lvl his/her skill up.